One type of power MOSFET transistor is known as a vertically diffused MOS (VDMOS) transistor. A transistor of this type is described in U.S. Pat. No. 4,631,564 to J. M. S. Neilson et al., issued Dec. 23, 1986, entitled GATE SHIELD STRUCTURE FOR POWER MOS DEVICE, and herein incorporated by reference.
In certain applications, power MOS transistors may be subject to direct short-circuits across a load to the power supply. In order to provide protection from the consequences of such a short-circuit, current limiting circuits have been utilized with power MOSFETs. An example of such a protection circuit is described in U.S. patent application Ser. No. 447,330, filed Dec. 7, 1989, entitled POWER MOSFET TRANSISTOR CIRCUIT, and herein incorporated by reference.
In the operation of such a protective circuit, it is herein recognized to be desirable to provide further protection from drain over-voltage conditions with an accurate clamp and to protect the power MOSFET from over-voltage transients. It is desirable that a protective circuit can be integrated into the power MOSFET and that it can be constructed using the same process as the power MOSFET.